Things that are forbidden when in ihram include removing hair from the head or body and clipping the nails. For each of these a fidyah must be offered. The individual has the choice between sacrificing a sheep; feeding six poor persons, giving each poor person half a saa‘; or fasting for three days. That is because Allah, may He be exalted, says (interpretation of the meaning):
“and do not shave your heads until the Hady reaches the place of sacrifice. And whosoever of you is ill or has an ailment in his scalp (necessitating shaving), he must pay a Fidyah (ransom) of either observing Saum (fasts) (three days) or giving Sadaqah (charity - feeding six poor persons) or offering sacrifice (one sheep)”
And because of the hadeeth of Ka‘b ibn ‘Ajrah (may Allah be pleased with him). When he needed to shave his head whilst in ihram, the Prophet (blessings and peace of Allah be upon him) said to him: “Shave your head, and fast for three days, or feed six poor persons, or offer a sacrifice.”
Narrated by al-Bukhaari, 4190; Muslim, 1201
It is not permissible to offer the fidyah in the form of cash, and it is not valid to do so, because that is not mentioned in the texts. Rather it must be given in the form of food, as enjoined by the Prophet (blessings and peace of Allah be upon him).
If the question is about removing something from the hair or nails after the month of Dhu’l-Hijjah has begun, in the case of one who wants to offer a sacrifice but is not in a state of ihram for Hajj or ‘Umrah, in this case even though it is haraam, no fidyah is required; rather he has to pray for forgiveness and repent.
See the answer to question no. 36567